In brushless DC motors, commutation states are defined for the mechanical alignments of the phase windings and magnets. These commutation states determine which motor phase is energized. Depending on the position of the motor, the torque applied to the motor can be either in the direction of desired rotation or can be in the opposite direction and many motors, such as a spindle motor on a hard disk drive, can be impaired or damaged if started in the reverse direction. If the rotor position is known before starting the motor, it is possible then to start the motor with no reverse rotation.
Determining the correct commutation state is especially difficult when the motor is at rest, because there is no motion induced back electromotive force (“BEMF”) to indicate the present commutation state.
Many detection systems exist that determine the position of the rotor in a brushless DC motor. For example, Hall effect sensing circuitry can be fixed to the motor and provides information about the present position of the rotor relative to an electrical period. The information provided by Hall effect sensing circuitry makes it possible to start the motor in the correct direction. Other motor designs may utilize a rotary encoder to directly measure the rotor's position. Once there is sufficient motion induced BEMF, the BEMF in the undriven windings can be measured to infer the rotor position. Thus, BEMF is useful to control the motor commutation after the motor has reached a sufficient RPM.